The field of the invention is thermistor controlled ovens used for maintaining a substantially constant oven temperature. Such ovens are commonly used in the electronics industry to maintain crystals used in oscillator circuits at a precise temperature.
Heating systems which cycle on and off between predetermined temperature limits are known but such systems exert no control when the oven temperature is within the predetermined limits. Also cycling systems have abruptly changing power requirements and can create unwanted noise signals on the power supply lines. Double ovens having two thermistors, one thermistor monitoring each oven temperature, are also used to maintain a precise oven temperature but waste space and electrical power. Cycling heating systems having an ambient temperature themistor and a control thermistor are known but the ambient thermistor variations merely change the cycling rate of the heating system in response to a change in ambient temperature.
Ovens using a closed loop thermistor controlled proportional heating system are known but such ovens do not maintain a constant oven temperature as ambient temperature varies unless they are mechanically compensated by adjusting a thermal resistance path such as described in U.S. Pat. No. 3,252,109. Such mechanically compensated ovens are bulky and waste oven space.
The present invention provides an electronic method of temperature compensating an oven instead of the previous cumbersome mechanical compensation method. Known systems have not normally distinguished between the control thermistor temperature and the actual oven temperature, and when the oven consists of a nonsymmetrical cavity, the temperature differences can be quite appreciable. The present invention provides a method for maintaining any point within a nonsymmetrical oven cavity at a constant temperature regardless of ambient temperature variations and no prior system provides such a method.